RFID is a technology that allows a wide range of objects to be identified, tracked and managed. RFID technology is based on the use of small radio tags or transponders and readers/encoders for connection to an information system. These RFID tags, which contain a unique code together with other additional information, can be read by the reader/encoder from a distance without contact or line-of-sight. Typical RFID tags are categorized as either active or passive.
Active RFID tags are typically powered by an internal battery and may include read/write functionality (i.e., tag data can be rewritten and/or modified). An active RFID tag's memory size may vary according to application requirements. In a typical active RFID system, the RFID tag might periodically transmit its data, including a part number and location to a central tracking database. The battery-supplied power of an active tag generally gives it a longer read range than a passive RFID tag. However, active RFID tags result in greater size, greater cost, and an operational life limited to about 10 years depending on operating temperatures and battery type, as compared to passive RFID tags. Additionally, active RFID tags are typically not suitable for indoor use, due to limitations and inherent unreliability of using traditional received signal strength indicators (RSSI) and time distance of arrival (TDOA) calculations.
Passive RFID tags operate without an internal power source. The passive RFID tags obtain operating power from the electromagnetic transmissions generated by the reader. Consequently, passive tags may be much lighter than active tags, less expensive, and offer a virtually unlimited operational lifetime. Passive tags are typically read-only and are programmed with a unique set of data that cannot be modified. However, passive tags may also be read/write tags that are initially programmed with a unique set of data and such data may be modified and updated at desired intervals. In order to operate, the passive tag reader must initiate the communication to the tag, and the tag then responds with its identifier. Typically, to obtain the location of the passive tag, the reader may use a combination of previous known location of the tag, RSSI and TDOA between the tag and reader, triangulation antenna gain measurement. Thus, passive RFID tags generally result in more accurate location of the tags as compared to active tags. Although cheaper and smaller than active tags, passive tags have shorter read ranges than active tags and require a higher-powered reader.
The advantage of RFID systems is the non-contact, non-line-of-sight nature of the technology. Tags can be read through a variety of substances, including metal, where barcodes or other, traditional optically read technologies would be impractical.
However, one disadvantage of RFID technology is that the tags need either a battery or need to be powered by the RFID reader in order to send the data stored therein. Batteries have limited life and must be recharged. There a variety of devices for recharging the on-board tag power storage, including capacitors, super capacitors, and rechargeable batteries. However, depending on the source used for powering the tag and the particular position and orientation of the tag, it may take different lengths of time for the tag to recharge and be ready to respond to the reader. Furthermore, each time a reader connects with a tag, the tag will require power to listen and respond, if needed, and thus using the internal power of the tag. Such a reader does not know how often it should talk to all those different sensor tags installed at different locations and the reader typically does not know the power state of the tag. A simple reader would be relegated to reading the tags only when all of the tags had charged up adequately to respond. The reader would not know the state of the tag so it would be set arbitrarily long to maximize opportunity to hear a given response. When this occurs, information from other tags that normally could be heard would possibly be lost. The read time could also potentially still be too short, but the simple reader would not know this and would fail to access the weak tag(s).